Page 158 - High Power Laser Handbook

126 Diode Lasers Semiconductor Laser Diodes 127

All designs rely on spatial multiplexing the output emission from
diodes stacked in a “staircase” formation. Each diode (usually) has a
corresponding FAC and slow-axis collimating (SAC) lens to collimate
and a mirror to point the emission beam. In some cases, the optical
path length is held constant so that a common SAC can be shared
among all emitters. The collimated output from all emitters is then
focused onto the output fiber with matching NA. The literature is rife
with various schemes to manipulate diodes attempting to strike a
balance among the following trade-offs: cost per watt, PCE, and foot-
print at a target output power and brightness level.
The vertical spacing of the diodes mounted on the staircase is lim-
ited by mechanical stack-up tolerances (±50 mm). To minimize this
penalty in aperture, the spacing may be increased tenfold to about
500 mm. With the output power chosen by the number of diodes
N times coupling efficiency (CE), the total aperture height h is simply
h = N × t, where t is the step height. The fiber NA is defined by the
application, so the coupling-lens focal length f is (h/2)/NA. Fast-axis
cl
(FA) magnification M is chosen less than or equal to 40 to avoid over-
y
filling the core while still remaining below the fiber NA. In the hori-
zontal axis, the emitter aperture and far-field divergence approximately
match the fiber-core diameter (105 mm) and NA (0.15), so the slow-
axis (SA) magnification M is near unity. The key formulae are sum-
x
marized simply as follows:
f = (h/2)/NA, where h = N × t (5.9)
cl
M = f /f ≤ 40 and M = f /f ~ 1 (5.10)
cl
y
x
cl
sac
fac
Based on the diode near-field and far-field characteristics and
other constraints to meet good CE, a practical limit of 5 to 7 diodes
can be combined. However, the brightness can be nearly doubled by
polarization-beam combining (PBC) two arrays of emitters. Recently,
IPG Photonics reported greater than 100 W with 50 percent PCE into
0.12 NA in a compact form factor (Fig. 5.23). 47
5.15 Qualification and Reliability

Package housings and their internal components undergo first-article
component qualifications, and the completed diode package must
also be qualified prior to product release. Telcordia GR-468-CORE is
regarded as a universal guideline that should be tailored to particular
41
deployment conditions and lifetimes. Among all endurance tests,
temperature cycling (–40 + 85ºC) and damp heat (85ºC/85% relative
humidity [RH]) are considered the most salient qualification tests,
especially for uncontrolled (UNC) environments.
Customers realize that qualification testing imposes a necessary
barrier to entry, but that qualification tests by themselves do not allow